Activate reproducible builds for XWiki

We should set project.build.outputTimestamp to activate reproducible builds. The build timestamp is automatically updated on release. Contrib parents will inherit it, which should be totally okay - in Maven 4, an inherited, fixed build timestamp will actually be the default. If an extension doesn't want this, it can always override the build timestamp.

This issue is not about ensuring that the build is actually fully reproducible, separate issues will be created for that. This issue is only about enabling the basic flag to instruct Maven to attempt producing a reproducible build.